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The index of dispersion as a metric of quanta – unravelling the Fano factor
Author(s) -
Fullagar Wilfred K.,
Paziresh Mahsa,
Latham Shane J.,
Myers Glenn R.,
Kingston Andrew M.
Publication year - 2017
Publication title -
acta crystallographica section b
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.604
H-Index - 33
ISSN - 2052-5206
DOI - 10.1107/s2052520617009222
Subject(s) - physics , index of dispersion , photon , fano plane , monte carlo method , detector , metric (unit) , fano factor , poisson distribution , statistical physics , quantum mechanics , statistics , mathematics , optics , geometry , poisson regression , population , demography , operations management , shot noise , sociology , economics
In statistics, the index of dispersion (or variance‐to‐mean ratio) is unity (σ 2 /⟨ x ⟩ = 1) for a Poisson‐distributed process with variance σ 2 for a variable x that manifests as unit increments. Where x is a measure of some phenomenon, the index takes on a value proportional to the quanta that constitute the phenomenon. That outcome might thus be anticipated to apply for an enormously wide variety of applied measurements of quantum phenomena. However, in a photon‐energy proportional radiation detector, a set of M witnessed Poisson‐distributed measurements { W 1 , W 2 ,… W M } scaled so that the ideal expectation value of the quantum is unity, is generally observed to give σ 2 /⟨ W ⟩ < 1 because of detector losses as broadly indicated by Fano [ Phys. Rev. (1947), 72 , 26]. In other cases where there is spectral dispersion, σ 2 /⟨ W ⟩ > 1. Here these situations are examined analytically, in Monte Carlo simulations, and experimentally. The efforts reveal a powerful metric of quanta broadly associated with such measurements, where the extension has been made to polychromatic and lossy situations. In doing so, the index of dispersion's variously established yet curiously overlooked role as a metric of underlying quanta is indicated. The work's X‐ray aspects have very diverse utility and have begun to find applications in radiography and tomography, where the ability to extract spectral information from conventional intensity detectors enables a superior level of material and source characterization.